1. Field of the Invention
The present invention relates to communication systems, and more particularly to apparatus and methods for data flow control in non-isochronous systems.
2. Description of Related Art
With the proliferation of computer systems to homes and businesses, communications technology has also advanced to accommodate the demands of businesses and consumers. Regional telephone companies, local exchange carriers, and internet service providers (ISPs) require greater bandwidth and faster communications to accommodate the needs of their customers. A vital component of the systems used by these companies is the network element connecting the host computer, such as an internet server, to multiple remote computers.
Access concentrators, such as Digital Subscriber Line Access Multiplexers (DSLAMs), are devices that typically provide termination ports for analog modem and integrated service data network (ISDN) dial access. Access concentrator functions generally include accepting multiple logical and/or physical streams of analog samples, formatting these samples into a sequence of frames, transferring the frames into the memory of a host system, providing application functions such as routing and authentication, and transferring the frames to a high speed network interface. To accommodate more users and data and provide superior systems, manufacturers and designers strive to increase the functionality, port densities, and port speeds of their access concentrators. Referring to FIG. 1, a typical conventional access concentrator 18 includes a local area network (LAN) or internetworking interface 51; a host controller 50; and a wide-area network (WAN) interface 20. The LAN interface 51 transfers data to and from the host controller 50 and a LAN. The WAN interface 20 performs several functions for transferring data between a WAN connection, such as a conventional T1/E1 cable, and the host controller 50, including physical termination, signal processing, protocol processing, and input/output (I/O) control for the host controller 50. Both the LAN interface 51 and the WAN interface 20 are connected to the host controller 50 via bus masters 52, typically comprising bus master direct memory access (DMA) interfaces, to delegate control of data transfers.
The WAN interface 20 generally includes multiple asynchronous modems 22, 24, 26, commonly comprising reduced power and board area versions of standard asynchronous client modems. With the multiple asynchronous modems 22, 24, 26, a set of modem I/O processors 28, 30, 32 are typically situated between the asynchronous modems 22, 24, 26 and the host processor 34 in order to transfer and, in some cases format, the asynchronous data, for example in conjunction with an asynchronous-synchronous point-to-point transmission (PPP) system. Each modem I/O processor 28 may handle the traffic for multiple, typically up to sixteen, asynchronous modems 22. ISDN data is routed through a set of separate controllers, such as multichannel HDLC controllers 36, 38, 40, instead of the modem cards 42, 44, 46 because the signal generated by each modem card 42, 44, 46 and the associated protocol processing can only be used for modem tasks.
Conventional modems transfer isochronous data to and from the host system and the WAN. For certain high speed modems, however, the data transfer rate on the host side may be different from the data rate on the line side. The line side for many systems transfers isochronous data at rates according to a selected clock signal. The host side, on the other hand, may transfer data non-isochronously data at rates that vary significantly. Many modern communications systems use data compression techniques to improve the effective transmission rate. The compression ratio, however, may vary according to the compression technique used and the data to be transferred. If the compression is performed by the access concentrator, data compression allows the host controller to transfer data to the access concentrator at a higher rate than the line side transmission rate. The rate at which the data is transferred from the host controller to the access concentrator, however, is typically limited by the lower rate line side transmission rate and the effective compression ratio. If the preceding data does not compress effectively, data transferred to the access concentrator may be lost before the access concentrator-can process and transmit it. On the other hand, if the data rate from the host controller to the access concentrator is reduced to ensure data integrity, the overall data transmission rate is reduced, regardless of the effectiveness of the compression method.
A data communications system according to various aspects of the present invention accommodates high transfer rates of non-isochronous data to and from a communications medium handling isochronous data. The system suitably includes an interface for transmitting non-isochronous data to and from a first system, such as a host system, and transmitting isochronous data to and from a second system, such as a WAN. The interface includes a clearto-send (CTS) signal generated by the system receiving the non-isochronous data. If the CTS signal is asserted, the sending system continues to provide data to the receiving system. On the other hand, if the CTS signal is deactivated, the sending system withholds the non-isochronous data for the relevant channel until the CTS is reasserted. In an exemplary embodiment, the CTS signal is an out-of-band hardware-implemented signal, which tends to provide optimal simplicity and speed.